The invention relates to a reflective active matrix liquid crystal display mixed with mixed twisted nematic and birefringent modes.
The present patent deals with reflective mode liquid crystal displays that combine the waveguiding effect and the birefringent effects of twisted nematic liquid crystals. By optimizing the various optical arrangements of the liquid crystal display such as the twist angle, the retardation and the polarizer angle, a series of mixed twisted nematicxe2x80x94birefringent (MTB) display modes has been invented.
There has previously been considerable activity in the study of reflective liquid crystal displays (RLCD). Broadly speaking RLCD can be divided into 2 categories: those that do not rely or polarizers and those that do. Examples of the former are reflective cholesteric displays and absorptive guest-host displays. The latter categories are necessarily nematic liquid crystal displays. These are displays that are based on polarization manipulation, as in ordinary twisted nematic LCDs. However, unlike ordinary LCD, there is only one front polarizer and the rear polarizer is eliminated.
The main applications for such RLCDs are in direct view displays with no backlighting, and in projection displays using crystalline silicon backplane with integrated CMOS drivers, or in reflective liquid crystal light valves (LCLV) in general.
Reflective nematic LCD has been investigated. One of the successful inventions is the so-called TN-ECB mode. A variation of this has been reported recently by Wu et al. It has a 90xc2x0 twist angle. There are many names given to display modes that operate on a combination of the waveguiding TN effect and the pure birefringent effect, e.g. the 45xc2x0, hybrid-field-effect (HFE) mode, the 63xc2x0, TN-ECB mode the 90xc2x0 mixed TN (MTN) mode, the self-compensated TN (SCTN) mode and the 52xc2x0 RTN mode.
A generalized picture of reflective twisted nematic LCD is disclosed herein that encompasses all of these mixed mode displays, and provide a method of optimizing them all at the same time. Many new operating conditions can be found that have not been reported and are the subject of the present invention.
These reflective liquid crystal displays can be fabricated on passive matrix or active matrix backplanes. The active matrix backplane can be fabricated on glass or on silicon wafers.
As shown in FIG. 1, the reflective nematic LCD consists of a polarizer, a twisted nematic liquid crystal cell, and a reflector, which can be part of a circuit in an active matrix device. The polarizer can either be a sheet type polarizer or a polarizing beam splitter as shown. In this invention, the PBS case is generally described as it is the most popular geometry for silicon microdisplays.
As discussed in the paper by H. S. Kwok, [xe2x80x9cParameter space representation of liquid crystal display operating modes, J. Appl. Phys., 80(7), 3687-3693 (1996)], all nematic RLCD modes can be represented in the parameter space diagram The parameter space in the case of twisted nematic RLCD is particularly useful, as it shows the relationship between the TN-ECB, MTN, SCTN and ECB modes. The reflectance R of the RLCD is a function of 3 major parameters: twist angle xcfx86, polarizer angle xcex1 between the polarizer and the input director of the LC cell, and the LC cell retardation dxcex94n where d is the cell thickness. The wavelength xcex always appears together with the retardation as dxcex94n/xcex in the Jones matrix. Therefore it can be treated as just a scaling of dxcex94n. Hence, if one of the 3 parameters (xcex1, xcfx86, dxcex94n) is fixed, R can be plotted as a function of the other two parameters in a 2D parameter space using contour lines.
FIG. 2 shows a series of parameter spaces for the RLCD, with xcex1 varying from 0 to 45xc2x0. A wavelength of 550 nm is assumed in the calculations. The contours indicate constant reflectance in steps of 0.1. The wells in FIG. 2 are the so-called TN-ECB minima. The center of the well corresponds to either maximum reflectance for crossed polarizers or minimum reflectance for parallel polarizers. For example, with a polarizing beam splitter (PBS) in the display, (xcex1, xcfx86, dxcex94n)=(0, 63.5xc2x0, 0.181 xcexcm) will give R=1. This corresponds to the first TN-ECB minimum. It is marked in FIG. 2 The SCTN mode and the MTN mode are also indicated in FIG. 2 for the appropriate xcex1.
Polarizer angles larger than 45xc2x0 are not depicted in FIG. 2. It is because that beyond 45xc2x0, the parameter space repeats itself, except for a reflection of the x-axis, i.e. the parameter space for xcex1=90xc2x0xe2x88x92xcex1 is the same as the one for xcex1, with xcfx86 changed to xe2x88x92xcfx86. From FIG. 2, it can be seen that there are 2 sets of operating modes for reflective LCD. One set of modes are the xe2x80x9cin-wellxe2x80x9d kind which correspond to the islands in the parameter space, such as the TN-ECB, MTN, SCTN modes. The other set of modes are the xe2x80x9cout-wellxe2x80x9d modes which are located outside the TN-ECB wells, such as the RTN, RSTN and HFE modes.
The xe2x80x9cin-wellxe2x80x9d modes are disclosed herein. It can be seen in FIG. 2 that the various TN-ECB minima move systematically in the parameter space as xcex1 is changed. In particular, the first TN-ECB mode with +xcfx86 is examined. It can be seen that this mode becomes the MTN mode at xcfx86=900xc2x0, xcex1=22xc2x0, then it becomes the SCTN mode at xcfx86=60xc2x0, xcex1=30xc2x0. Finally, this first TN-ECB minimum becomes the true ECB mode at xcfx86=0xc2x0 and xcex1=45xc2x0.
The situation is clearly shown by a plot of the trajectory of the center of the first TN-ECB minimum for the +xcfx86 case as shown in FIG. 3. In this plot, xcex1 goes from 0 to 45xc2x0 in steps of 5xc2x0. It can be seen the first TN-ECB minimum first moves out and then towards the y-axis. The retardation increases monotonically as xcex1 increases. The maximum twist angle reaches 70.2xc2x0 at a polarizer angle of 15xc2x0. FIG. 4 is a similar plot of the 0.9 reflectance contours for xcex1 ranging from 0 to 90xc2x0, again in steps of 5xc2x0. This plot is different from FIG. 3 because we also include xcex1 from 45xc2x0 to 90xc2x0. As can be seen from FIG. 4, as xcex1 goes from 45xc2x0 to 90xc2x0, the originally xe2x88x92xcfx86 TN-ECB minimum moves into the positive xcfx86 side, thus forming a complete loop in the parameter space. This is more easily seen in a parameter space showing both positive and negative twists (FIG. 5).) Notice that the parameter space for xcex1 and xcex1+90xc2x0 are identical so that a complete trajectory is formed in FIG. 5 as a goes from 0 to 90xc2x0. FIG. 3 indicates that for twist angles from xe2x88x9270xc2x0 to +70xc2x0, there always exists 2 first order TN-ECB minima at different polarizer angles, one with a smaller dxcex94n value and one with a higher dxcex94n value.
The operating points of the MTN mode, the TN-ECB mode and the SCTN mode are also indicated in FIG. 4. Thus FIG. 4 unifies the entire picture for the TN-ECB, the MTN and the SCTN modes. They all operate with a combination of polarization rotation (TN) and birefringence (ECB) effects. They differ by a rotation of the polarizer relative to the input director, or, in other words, by the proportion of TN to ECB effects. Therefore, it should be possible to perform an optimization of these modes in a general sense, allowing for variations of all 3 parameters simultaneously.
The nomenclature of these nematic reflective LCDs will now be defined. Since all of these modes operate with a combination of TN effect and ECB effect, they can be called a hybrid mode or a mixed mode. They have been called TN-ECB, MTN, SCTN or HFE in the literature. Instead of calling them the TN-ECB/MTN/HFE mode, such LCD operating modes are hereinafter referred to as the generalized mixed TN-birefringence mode, or MTB mode in short.
In the optimization of the MTB mode, it can be assumed that high reflectance is desirable. If the desired reflectance or light efficiency is set to be 0.9, then the solution will be bound by the 0.9 reflectance contours depicted in FIG. 3. This limits the parameter space tremendously. Alternately, if the desired reflectance is set to be 0.7, then the solution space opens up even larger, and includes twist angles up to 100xc2x0. The 0.7 reflectance contour is plotted in FIGS. 4-21 for polarizer angles in increasing step of 5xc2x0. This represents all the possibilities for the MTB mode. It is noted that all the previously reported modes are represented in FIGS. 4-21. For example, the 90xc2x0 MTN mode of Wu et al is included in FIG. 8. The SCTN mode of Yang is included in FIG. 10.
FIG. 22 is a composite of all the R =0.7 contours in FIGS. 4-21. It is seen that there are always 2 distinct MTB modes for xcfx86 less than 70xc2x0. Usually the solution with smaller dxcex94n is less wavelength dispersive than the larger dxcex94n one. However, a large dxcex94n is desirable from a cell making point of view.
Table I shows the normal operating brightness of the RLCD under various polarizer geometries and using different xe2x80x9cin-wellxe2x80x9d and xe2x80x9cout-wellxe2x80x9d modes. It can be seen that normally white(NW) and normally black (NB) operations can be achieved in both cases, depending on the polarizer geometry. The choice of polarizers has a profound effect on the optimization of the RLCD. In order to have excellent contrast, the dark state should be made as dark as possible. This is conveniently satisfied by the homeotropic alignment of the LC under high voltage bias where dxcex94n =0 (the x-axis in FIG. 2). This homeotropic state is nondispersive and should be used as the dark state whenever possible. For the MTB modes, which correspond to the centre of the wells in FIG. 1, the homeotropic state can be used as the dark state if a PBS is used. The display will then be normally bright.
However, if a parallel polarizer geometry is used, the high voltage homeotropic state is the bright state and the MTB wells become the dark state. That is undesirable since the MTB wells are dispersive and cannot be made very dark. Hence the contrast will be poor. So for direct view using MTB operation, a quarterwave retardation film has to be used to reverse the bright and dark states. Another solution is to make use of the xe2x80x9cout-wellxe2x80x9d modes such as the RTN and HFE modes as the bright state. In the case of the RTN, the dark state can be made reasonable dark by further optimization of the polarizer angle
Table II summarizes some of the new MTB modes provided using the parameter space method. These modes have reasonably good reflectance and low dispersion. All of them have not been presented before.